# std::rethrow_if_nested

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< cpp‎ | error

Defined in header `<exception>` |  |
template< class E > void rethrow_if_nested( const E& e ); |  | (since C++11) (constexpr since C++26)

If `E` is not a polymorphic class type, or if std::nested_exception is an inaccessible or ambiguous base class of `E`, there is no effect.

Otherwise, performs

    if (auto p = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
        p->rethrow_nested();

### Parameters

e | - | the exception object to rethrow

### Notes

Unlike many related functions, this function is not intended to be called with a std::exception_ptr but rather an actual exception reference.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_exceptions` | `202411L` | (C++26) | constexpr for exception types

### Possible implementation

    namespace details
    {
        template<class E>
        struct can_dynamic_cast
            : std::integral_constant<bool,
                  std::is_polymorphic<E>::value &&
                  (!std::is_base_of<std::nested_exception, E>::value ||
                    std::is_convertible<E*, std::nested_exception*>::value)
              > {};

        template<class T>
        void rethrow_if_nested_impl(const T& e, std::true_type)
        {
            if (auto nep = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
                nep->rethrow_nested();
        }

        template<class T>
        void rethrow_if_nested_impl(const T&, std::false_type) {}
    }

    template<class T>
    void rethrow_if_nested(const T& t)
    {
        details::rethrow_if_nested_impl(t, details::can_dynamic_cast<T>());
    }

### Example

Demonstrates construction and recursion through a nested exception object.

Run this code

    #include <exception>
    #include <fstream>
    #include <iostream>
    #include <stdexcept>
    #include <string>

    // prints the explanatory string of an exception. If the exception is nested,
    // recurses to print the explanatory string of the exception it holds
    void print_exception(const std::exception& e, int level =  0)
    {
        std::cerr << std::string(level, ' ') << "exception: " << e.what() << '\n';
        try
        {
            std::rethrow_if_nested(e);
        }
        catch (const std::exception& nestedException)
        {
            print_exception(nestedException, level + 1);
        }
        catch (...) {}
    }

    // sample function that catches an exception and wraps it in a nested exception
    void open_file(const std::string& s)
    {
        try
        {
            std::ifstream file(s);
            file.exceptions(std::ios_base::failbit);
        }
        catch (...)
        {
            std::throw_with_nested(std::runtime_error("Couldn't open " + s));
        }
    }

    // sample function that catches an exception and wraps it in a nested exception
    void run()
    {
        try
        {
            open_file("nonexistent.file");
        }
        catch (...)
        {
            std::throw_with_nested(std::runtime_error("run() failed"));
        }
    }

    // runs the sample function above and prints the caught exception
    int main()
    {
        try
        {
            run();
        }
        catch (const std::exception& e)
        {
            print_exception(e);
        }
    }

Possible output:

    exception: run() failed
     exception: Couldn't open nonexistent.file
      exception: basic_ios::clear

### See also

nested_exception (C++11) | a mixin type to capture and store current exceptions (class)
throw_with_nested (C++11) | throws its argument with std::nested_exception mixed in (function template)

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